In imaging methods as e.g. electro(photo)graphy, magnetography, ionography, etc. a latent image is formed that is developed by attraction of so called toner particles. In DEP the so called toner particles are image-wise deposited on a substrate. Toner particles are basically polymeric particles comprising a polymeric resin as main component and various ingredients mixed with said toner resin. Apart from colorless toners, which are used e.g. for finishing function, the toner particles comprise at least one black and/or coloring substances, e.g., colored pigment.
In the different imaging methods, described above, the toner particles can be present in a liquid or in a dry developer composition.
In most cases the use of dry developer compositions is preferred. The main advantage of using a dry developer composition resides in the absence of the need to eliminate the liquid phase after development. The avoidance of the need to evacuate (mainly organic) liquids is desirable both from an economical standpoint and from an ecological standpoint.
The major problem in using dry development systems instead of liquid development systems has longtime resided in the fact that the dry toner particles could, in an economically sound way, only be produced with large particle sizes. The large particle size of toner particles has limited the image resolution that was attainable by dry development systems when compared to liquid development systems. The preparation methods for toner particles have however evolved in such a way that dry development systems can now be used for high resolution imaging.
Further advancement in technology, e.g. the amount of gray levels that can be contained in an image, the use of colored particles, etc. has made the imaging techniques, described above, reliable enough to realize high quality colored images, comparable in quality to the quality of offset printing and of conventional (silver halide) color images with the additional advantage that these images can be realized on different substrates such as paper, transparencies, plastics etc. . .
It moreover offers the possibility to create images at high speed and double sided and combining high resolution text with images comprising a large amount of gray levels. It offers with respect to thermosublimation imaging systems excellent resolution and fine gray rendition at high speed.
However, in all techniques using dry particulate material to form an image, the images are built up by application of particulate marking elements in multiple, superimposed layers onto the final substrate. The problems associated with multiple, superimposed layers of particulate marking particles that are in one way or another fixed on a substrate are manifold, not only with respect to image quality but also with respect to image stability and with respect to mechanical issues. E.g. superimposed layers can only give brilliant mixed colors when the different particles having a different color are intimately mixed. When fusing and flowing of the particles are sub-optimal, some microporosity remains in the image, giving rise to light scattering and to mixing white light with the image colors diminishing the color intensity (saturation). When the flow of the molten toner is not sufficient, some relief pattern remains in the image, which gives rise to differences in gloss. When the flow of the molten toner is not sufficient, the image, formed by superimposed layers of toner particles, is present as a thick layer. Thick layers will give more mechanical stress in the image, inducing curl, cracks, etc. . . , thick layers are more fragile so that rubbing, folding etc. . . , make the final image more sensitive towards deterioration after processing.
It is thus necessary that in the fixing step of the image on the final substrate means should be found to mix the different superimposed toner particles intimately and to diminish the overall thickness of the image layer.
In most of the fixing steps of toner particles heat is involved in one way or another (e.g. pressure and heat fixing, oven fixing, IR (infrared) radiation fusing, etc.).
It has been recognized that by modifying the toner resin, it was possible to produce toner particles which made it possible to have an image with well mixed colors and with a thin image layer.
The main property needed in a toner resin, which makes it possible to have an image with well mixed colors and with a thin image layer, is a good melt fluidity (i.e. low melt viscosity) at quite low fixing temperature.
It is possible to lower the melt viscosity during the fusing process when using fairly high fixing temperatures. This means compromising on power consumption or on copy speed for a given toner resin. Moreover the usable fixing temperature is limited by the thermal properties of the final substrates whereon the toner particles are fixed. Too high fixing temperature can result in yellowed paper, wrinkled transparencies, etc.
The other possibility to lower the melt viscosity of toner particles lies in the design of the toner particles, especially in designing the toner resin with respect to its melting point. The melting `point` of the toner, depending mainly on the melting point of the toner resin, however can not be lowered without lowering in a pronounced way the glass rubber transition temperature (Tg) of the toner resin. Lowering that temperature however induces the deterioration of the toner marking particles and marking system due to the fact that the toner particles will be, at room or slightly higher temperature, too weak to overcome deformation, smearing, etc. by the mechanical forces exerted on the toner particles in the development units.
In most imaging systems, now available on the market place, the toner particles comprise more or less sophisticated toner resins, but the compromise between Tg and melt temperature and melt viscosity is in most commercial toner particles rather fragile. In most systems, still an appreciable amount of fusing energy is needed and high fusing temperatures are not uncommon, even when Tg of the toner resin is chosen marginally low, reducing developer lifetime and image quality upon prolonged use.
In order to optimize the compromise between Tg and melt viscosity of toner resins, more sophisticated polymers are described as being useful as toner resin. The major concept is to use a multi-component macromolecule as toner resin. In such a multi-component macromolecule, polymeric moieties, having different properties, are combined to give a final macromolecule showing a compromise between Tg and melting behavior that can depend on the nature of the polymeric moieties that are combined in the same macromolecule.
In NL 73-00096 a crystalline multi-component macromolecule is disclosed, comprising an amorphous backbone and a high content of crystalline polymeric side-chains. The amount of crystalline side-chain is chosen such as to produce a final crystalline macromolecule. Such a crystalline macromolecule shows a sharp melting point, ranging at most over 10.degree. C. Although the macromolecules, disclosed in NL 73-00096, show, when used as toner resin, advantages over pure crystalline homopolymers (e.g. toners comprising said macromolecules have lower dark decay and low conductivity), they tend to have the typical crystalline fracture mechanics. Because of these fracture mechanics, toner particles comprising said macromolecules and produced by melt kneading and milling, have flat, good aligning fractured surfaces, these surfaces giving rise to strong cohesion, strong adhesion, low flow and poor individual particle behavior. These problems make toner particles, comprising the above mentioned macromolecules, of limited use for high resolution halftone imaging.
The crystallinity of said macromolecules diminishes greatly the compatibility of the macromolecules with a host of toner ingredients. This makes them also less suited as universal toner resin since the use of them limits the freedom for choosing other toner ingredients.
In EP-A 099 140 a multi-component macromolecule comprising immiscible crystalline and amorphous blocks is taught as toner resin. The crystalline blocks form the continuous phase of the macromolecule and have a melting point between 45 and 90.degree. C. The amorphous blocks have a Tg at least 10.degree. C. higher than said crystalline blocks. The crystalline blocks are present in the macromolecule for at least 65% by weight and for at most 95% by weight. Also this macromolecule is basically crystalline and shows the same drawbacks of crystalline macromolecules as explained above.
In EP-A 220 319 the use as toner resin of a block polymer comprising amorphous and crystalline blocks has been described. It is preferred that both the amorphous and crystalline blocks are polyester resins.
In JP-A 01 268,722 a preparation method for polyesters, that are intended to be used as binder in paints, has been described. The polyester is prepared by copolycondensation with polyolefins in low concentration, about 2% by weight.
In EP-A 477 512 and corresponding U.S. Pat. No. 5,158,851 and U.S. Pat. No. 5,238,998 it is disclosed to use as a toner resin a complex macromolecule having a general formula (A--B).sub.n wherein n is at least 2 and A and B represent different polymeric segments. It is intended to form a liquid glass macromolecule. In this type of polymers, at least one of the polymeric segments is a glassy segment with a Tg higher than room temperature, preferably higher than 50.degree. C. and the other segment is a liquid polymer having a Tg lower than room temperature. In these resins the mobility of the liquid segments is frozen by the glassy segments at a temperature below the Tg of the latter. The mobility is high at temperatures above said Tg. Although the compromise between Tg and melting behavior of these macromolecules is better than with the mainly crystalline macromolecules mentioned above, there are still some problems associated with the use of said macromolecules as toner resin. Some of these problem are: The preparation of said macromolecules is quite complex, the melt flow behavior is Tg controlled which is an inherently unsharp process (the crystalline macromolecule referred to above exhibits a sharp melting behavior). The choice of and the amount of useful glassy segments to be combined with the liquid segments is limited, due to the fact that the liquid segments have a strong softening influence on the macromolecule and that the glassy segments must have an sufficiently high Tg to give an acceptable Tg to the complex macromolecule. The Tg of the complex macromolecule has to be high to overcome deformation, smearing, etc. by the mechanical forces exerted on the toner particles in the development units. But by increasing the Tg of the complex macromolecule by using a fair amount of glassy segments with high Tg the melt flow characteristics are worsened and the complex macromolecule shows again a labile compromise between Tg and melt flow characteristics.
In U.S. Pat. No. 3,967,962 it is disclosed to have a segmented copolymer as toner resin, said segmented copolymer comprising one crystalline or crystallizable polymeric segment chemically linked to at least one amorphous isomeric polymeric segment. The segmented copolymer has a Tg lower than 20.degree. C. and a melting point higher than 45.degree. C. It is preferred that the melting range is between 45 and 150.degree. C. The segments, forming the segmented element, need to have low Tg : the crystallizable segment has a Tg between 20.degree. C. and -100.degree. C., the amorphous segment need to have a Tg lower than 45.degree. C.
It is the object of the disclosure in U.S. Pat. No. 3,967,962 to provide a toner resin with low melting point and good hot off-set properties. Therefore it is proposed to combine a crystalline (or crystallizable polymer) with an amorphous polymer into one complex macromolecule that has to remain crystalline. It is clear that a toner comprising such a toner resin will have desirable melt viscosity characteristics, but due to the low Tg, problems with deformation, smearing, etc. by the mechanical forces exerted on the toner particles in the development units will occur to a large extent.
In U.S. Pat. No. 5,166,026 a copolymer of eicosene with styrene is disclosed to give a toner resin with an acceptable compromise between Tg and melt flow characteristics.
In U.S. Pat. No. 5,026,621 a block copolymer, comprising fluoroalkylacrylester moieties are disclosed as useful as toner resin for the cited reasons.
The disclosures, mentioned earlier on, are mainly concerned in modifying crystalline polymers and are also, apart from U.S. Pat. No. 3,967,962, mainly concerned with addition polymers. The teachings do not extend to the use of amorphous modified polycondensation resins, although amorphous polycondensation resins do offer advantages when used as toner resin.
The addition polymers are reasonable good binders but do not offer the major advantages of the polycondensation type resins, especially of polyester resins. Polyester type resins offer a combination of good basic electrostatic properties, good optical clearness and good dispersing power for toner ingredients. Therefore these polyester type resins are the preferred resins for high quality applications, such as full color applications. Polyesters, moreover, offer already in the amorphous state a good starting point for a fair balance between Tg and melt behavior. However teachings on the manufacture of amorphous complex polycondensation macromulecules and their use as toner resins have not been found.
The process of manufacture of the addition polymers, described in the documents mentioned above, is not an easy one or does not deliver well defined complex macromolecules.
The methods for the production of complex addition macromolecules are typical block and graft procedures known in the art. Typical methods are radical polymerization, anionic or cationic polymerization, group transfer polymerization as described in, e.g., Journal of Coatings Technology Vol. 59, No 752 pages 125 to 131 and EP-A 068 887, pseudo living free radical polymerization as disclosed in, e.g. WO 94/11412 and latex grafting. Although all these methods are useful for making block or graft polymers, the practical use is often limited mostly due to the complexity and cost of the production process. In ionic polymerization methods and in group transfer polymerization methods, the purity of the monomers and the solvents, the low reaction temperature and high vacuum needed limit the possibilities of this technique. The use of organic solvents makes these methods also less attractive from an ecological point of view.
In pseudo living free radical polymerization methods the main drawback lies in the long reaction times, that can extend from one day to several weeks.
With latex grafting methods it is almost impossible to have pure block polymers, since in most cases there will always be a part of homopolymer present.
The synthesis of blockcopolymers, as disclosed in U.S. Pat. No. 3,967,962, by the coupling of mono and/or bifunctional polymer segments to each other by the reaction of a bifunctional low molecular weight product, preferably a diisocyanate, is a simpler method to form blockcopolymers. Unfortunately this method delivers a mixture of final blockcopolymers, of which the structure can not easily be predetermined, and of homopolymers consisting of several equal segments linked together. Moreover after completion of the reaction, the polymer mixture has to be purified and the unreacted diisocyanate has to be extracted by organic solvents, which again is undesirable from an ecological point of view.
In EP 298 214 a reaction, presumably an ester interchange reaction, is described to provide a macromolecule comprising polyester and polyacrylate moieties. The product formed seems to be rather a new composition (mixture) wherein the two components are intimately admixed instead of a new macromolecule.
In U.S. Pat. No. 5,578,409 and the European equivalent EP-A 606 873 it is disclosed to use polyester modified by a long chain aliphatic alcohol or carboxylic acid having at most 101 carbon atoms.
There is thus still a need to have resins available that combine relatively high Tg with good melt fluidity (low melt viscosity) at fairly low melting temperature and that are mainly modified complex polycondensation macromolecules, especially modified polyester resins. There is moreover still a need for an easy, simple and cost-efficient procedure for manufacturing such complex polycondensation macromolecules that makes it also possible to taylor a macromolecule to the particular needs of toner particles to be used in each particular imaging system.